An underground water main ruptured in downtown St. Paul late Friday, Feb. 8, sending an estimated 1.75 million gallons of water coursing through the streets of Lowertown, disrupting service and prompting the city to test the area's tap water.

The break in the 20-inch, 61-year-old main occurred around midnight on Wall Street, between Fifth and Sixth streets, with water cascading down Wall and Broadway streets toward Kellogg Boulevard for two hours.

For nearly 12 hours after the break, the city advised residents in affected areas not to drink the tap water until testing was completed.

By 3 p.m. Saturday, the city changed its advisory from a "do not drink" to "boil water" advisory after finding no harmful chemicals in water samples.

WHAT IS A BOIL WATER ADVISORY?

At 3 p.m., St. Paul Regional Water Services lifted its "do not drink" water advisory and replaced it with a "boil water" advisory. Initial tests raised no concern about volatile organic compounds, a class of chemicals that can be harmful in water.

Under a boil water advisory, residential customers can use water for cooking or drinking after it has been boiled for three minutes. Commercial dishwashing systems can also be used because they heat the water enough to kill bacteria.

The boil water advisory won't be lifted until more samples can be analyzed for potentially harmful bacteria. Those tests are expected to take about 18 hours to complete, according to water authorities.

Bacteria is still a concern, but restaurants can use commercial dishwashing systems because the water is heated to a temperature that kills bacteria. Residential customers should still avoid dishwashing, though, because their water likely does not get hot enough.

On Saturday morning, deposits of grit and debris had accumulated near the St. Paul Farmers' Market site, and water still flowed from the site of the break into a stormwater drain on Wall Street -- though utility crews had the break isolated by 3 a.m.

On Saturday afternoon, city production distribution manager Jim Graupmann said crews had a chance to inspect the pipe and were attempting to determine the cause.

"We may not know, ever, but we'll take a look and see," he said. "Usually, it's corrosion. It's weakened to the point where (the pipe) can't stand the pressure that's in the main. It could be a manufacturing defect, but that's more rare."

The pipe dates back to 1952, Graupmann said, but added that isn't older than average for the system. The last time he recalled a break of this magnitude was in 1991.

A large swath of St. Paul had its water service affected by the break.

Wall Street between Fifth and Sixth streets in downtown St. Paul, after a water main break left entire swaths of the area without power early Saturday morning. (Pioneer Press: Kraig Odden)

The exact number of affected customers is unknown, but homes and businesses east of Interstate 35E, west of U.S. 52, south of University Avenue and north of the bluff line experienced water supply and pressure issues, according to Graupmann.

That area includes St. Paul's Lower West Side, downtown and Lower West Seventh neighborhoods.

There were reports of water problems from the West End neighborhood around Western Avenue and Sturgis Street, but Graupmann said city crews there did not find any infrastructure problems.

Water pressure was restored to most of the affected area by 3 a.m., city council member Dave Thune said.

Water rushes across Kellogg Boulevard and Wall Street in downtown St. Paul in the early hours of Feb. 9, 2013, after a water main broke. (Pioneer Press: Kraig Odden)

The precautionary measures on drinking water are required by the Minnesota Department of Health because the ruptured main caused a loss of water pressure in the system, which could have allowed harmful bacteria to enter the water supply -- though there was no evidence of that happening.

It was a month ago when a large water main break hit downtown Minneapolis, disrupting service for a large portion of the city for several hours and a few buildings for several days.

On Jan. 3, a 36-inch pipe ruptured, sending as much as 14 million gallons of water into the streets.

The water main was severed after a contractor working on a $70 million retail-apartment complex struck the pipe with a backhoe.

A water main break on Wall Street between Fifth and Sixth streets in the early hours of Feb. 9, 2013, caused flooding and water service outages. (Pioneer Press: Kraig Odden)